Zastin is Lala's personal bodyguard and the best swordsman on Deviluke. He is also the commander of Planet Deviluke's royal family bodyguards and the assistant chief of Studio Saibai, the manga company owned by Rito's dad, a title he likes to brag about.

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Zastin has greyish hair and blue eyes. He often seen wearing his demonic-like armor and also wears a cape (sometimes he does wear T-shirt and sweatpants while working on manga). Zastin always carries a big green energy sword, which the blade can disappear to put away easily. Zastin's tail is thin and very scorpion-like in appearance. He is also shown to have quite a muscular upper body.

Zastin is quite formal with those around him, especially with Rito, who is engaged with Lala. He can also be very strict to those less tolerable, mostly to the Deviluke princesses. Despite his skill with the sword, Zastin has a horrible sense of direction, resulting in him getting lost very often (one of the running gags in To Love-Ru), though this makes sense as he is on another planet. Rito states he can be a bit dumb, such as when he brags about his title as assistant chief of a manga artist to his enemy, and he is easily persuaded (Rito once stated he's a gullible idiot.) He also seems to have extremely bad luck, and keeps getting run over by trains and cars (another running gag of To Love-Ru). He also hates admitting defeat or loss as shown in chapter three. Zastin enjoys doing his job as a mangaka, more so than his job as bodyguard (ironically he seems to be a lot better at it). Zastin can be seemingly oblivious. Interestingly whenever he is shown trying to protect Lala, he fails at it horribly.

Back at his home planet Deviluke, Zastin always looked after Lala and her sisters while their parents weren't present. He would also put up with Lala's experiments, especially when they would cause a ruckus around the royal palace.

Zastin has had a history with the assassin, Black, whom he knows was responsible for destroying the organization, Eden. Zastin and Black have been both allies and enemies in the past.

Zastin came to Earth to bring Lala back to their planet but was defeated by her invention Go-Go Vacuum-Kun. In the Manga, at first, Zastin disapproves Rito, deeming him weak and subsequently seeks to test his strength. However, once Rito exclaims his opinion of marriage in self-defense, Zastin is swayed by his words (though not in the way Rito intends) and summarily approves of him, believing he truly understands Lala's feelings. While he lets her live on Earth and allows her being engaged to Rito, he also starts to live on Earth to continue his job as her bodyguard.

When Lala's sisters, Nana and Momo, ran away from home for the second time to avoid studying, Gid instructs him to bring them back, but he ultimately fails after being overpowered in battle.

He is currently working as the assistant chief to Saibai Yuuki. The funds he gets for his duties are handed to Lala for her use. Zastin has shown a keen interest in becoming a mangaka professionally, and has even won an award for his own manga. He has two assistants that work under him, simply known as Maul and Smutts.

In To Love Ru Darkness, he was made aware about Mea's abilities and questions if she is really to be trusted. After the incident when Golden Darkness unwillingly activates her Darkness transformation, and put the whole planet and the Devilukean sister in danger, Zastin began investigating the Transformation Weapon girls. After gathering information from his old acquaintance, Black, who destroyed the organization that created the Transformation Weapons, Zastin came to realize that the rogue Transformation Weapon, Nemesis, was plotting to destroy the political system established by Deviluke at the end of the Galatic War, thus starting another war across the universe. But being unlikely able to defeat her, Zastin instead teleports her, himself, and Rito Yuuki to a cyberdimension, where they are confrontated by the king of Deviluke himself, Gid Lucione Deviluke.

Devilukean Physiology: Zastin has incredible physical abilities which are common traits among Devilukeans.

Devilukean Super Strength: Being from the race of Devilukeans, Zastin is much stronger than any other known race. One time he easily picked up Saki Tenjouin's limousine when it was stuck in a ditch.

Devilukean Durability: Zastin also has been shown to have great durability, most notably when being hit by moving vehicles like a cars or a trains.

Swordsmanship: Zastin is shown to be quite skilled at using a sword and is known as the greatest swordsman on his planet. He could even fight Yami to a tie during their first encounter; however, most of the time, his bad luck takes toll on him and results in miserable failure.

Field Resourcefulness: Zastin is also capable of using the field advantage as seen when he lured Yami to a moving train, but can be really clumsy sometimes.

Manga Illustration: Zastin also happens to be a really good at drawing manga, and even received an award for drawing his own.

Out of everyone in the series, Zastin looks up to and respects Gid the most as the King of the planet Deviluke and Emperor of the entire Milky Way Galaxy. He is one of the bodyguards who protect the king's family members and would get mad at anyone if they disrespect any of them.

As the first born and heir to the Planet Deviluke, Zastin shows respect to Lala. Despite his loyalty to her, he often seems to have a miserable time serving her. He encouraged and urged her to behave more properly like a princess, and was forced to follow her when she ran away from home. And many of the troubles caused by Lala would end up giving him trouble as well. It was once said that when Lala was younger, Zastin was the guinea pig for her inventions and tried to run away, only to be tracked down and caught by her robotic search dog, Sniff Sniff Trace-kun.

As daughters of the King of Deviluke, Zastin expects them to follow orders as Princesses of Deviluke, but instead they follow their sister to Earth to escape from their studies, which also causes him just as much trouble as Lala did even after he failed to bring them back.

At first he didn't accept Rito marrying Lala and becoming the heir to the throne and the future King of Deviluke and Emperor to the Galaxy, stating it's not a position for weaklings and battles him so he would step down. However, after hearing Rito's speech about how marriage would only work if two people love each other, he mistook this as a right that he is the only one suited for Lala. Touched by this, he then accepts Rito's right to marry her, which Rito did not originally intend.

Afterwards Zastin started showing respect to Rito and even called him with a polite honorific "Muko-Dono" (roughly translates to "Master Groom"). He occasionally would help Rito out when he is stuck in a difficult situation involving something from space and would go to great lengths to protect him (even though he often fails in the process).

Saibai is a manga artist and Rito's and Mikan's father. Zastin works for him as his chief assistant. The previous assistants quit after Lala's invention helped them work much faster but severely exhausted them. Zastin, along with Maul and Smutts, agreed to take their place in helping Saibai complete his mangas and have been working there ever since (even though they are supposed to be Lala's bodyguards).

Saki became smitten for Zastin ever since he helped lift her car out of a hole in the road. While Saki made a few attempts to get his attention, most of those times, one mishap after another would throw her into an embarrassing situation in front of him, such as revealing her panties to him, which he tries to politely deny seeing, which does little to convince or console her.

While Zastin is formal and polite towards her as he is to anyone else, it is unknown what he feels towards her, but there is little evidence that he has romantic feelings towards her. Saki appears to be the only female character (near his age) with affections for another male character besides Rito, which may cement her as one of few girls who is genuinely not a love interest for Rito.

Maul and Smutts are also Devilukean bodyguards like Zastin. Zastin seems to be incharge of them as the two are never seen without him and seem to follow everything he says. They also work under Saibai Yuuki as Mangaka's and also seem to be enjoying it like Zastin.